In most cases, you should lay out your application in portrait view. Users tend to hold their BlackBerry smartphone in one hand, and therefore, typically use their smartphone in portrait view.
Sometimes, landscape view can offer distinct benefits and provide a better experience for all-touch BlackBerry smartphone users. Take movies and games, for example. Movies are created to be viewed in wide screen, and games often require a lot of two-handed interaction.
In some cases, you might lay out your application in both portrait and landscape view. With this approach, you can provide a slightly different focus in each view to enrich the user experience. For example, you could display a basic calculator in portrait view and a more advanced version in landscape view.